How do species evolve?

Transcription

1 BIOL2007 THE ORIGINS OF SEIES Kanchon Dasmahapatra Biodiversity 1.5 million described species Maybe as many as 30 million species overall How does speciation happen? Speciation genetic divergence within a population Need to prevent/reduce gene flow for species to form. rocess is usually slow. Genetic divergence takes time How do species evolve? Allopatric speciation: vicariance Geographic modes of speciation allopatric parapatric sympatric auses of speciation random forces (mutation and drift) deterministic forces (natural selection) Barrier to gene flow (mixing) 1

2 1) Allopatric speciation a) Vicariance Range of a species split in two. No gene flow. Divergence in separate populations Alpheus snapping shrimps Eventually, barriers erode and maybe secondary contact. Three outcomes are possible: 1) Little divergence: broad or narrow hybrid zone. 2) Hybrid inviability/sterililty 3) May have already become separate species Snapping shrimps Alpheus snapping shrimps across Isthmus of anama b) Allopatric speciation - the founder effect A speedier allopatric mechanism was suggested: "founder effect" Mayr (1954): 1) founders, take small fraction of available genetic variation (genetic drift). 2) Strong selection leading to adaptation to different ecological in new home. 2

3 Hawaiian Drosophila Hawaiian Drosophila 500 named species other known but undescribed species Start of Hawaiian Drosophila radiation dated to 20 Mya Similar speciation patterns for Hawaiian snails and crickets arson 1983 Genetics 2) Sympatric speciation Genetic divergence of populations inhabiting the same geographical area Speciation must occur under very high levels of gene flow. Selection causing speciation must be very strong. Host races in the apple maggot, Rhagoletis pomonella Native host: hawthorn Became apple pest in 1860s, due to a host switch Apple-eating form quickly spread all over E. USA Reduced gene flow of 6% between races If this kind of sympatric evolution (or almost-speciation) can occur in a few tens of years, could be an extremely important over geological time 3

4 ichlid fish in African Great Lakes ichlid fish in crater lakes Schliewen et al (1994) Nature Tanganayika Victoria Malawi Lake Victoria Malawi Tanganayika Area / km 2 68,600 29,600 32,900 No. endemic species ~ One or two colonisations in Lake Victoria and Malawi Specialised feeders Sexual dimorphism: males often brightly coloured Area Endemic Lake / km 2 species Barombi Mbo 4 11 Bermin mtdna trees 3) arapatric speciation auses of speciation Speciation due to non-random mating of a population within a continuous geographic area. Difficult to prove parapatric speciation. In any case allopatry only superficially different from parapatry; gene flow is always somewhat restricted random forces (mutation and drift) deterministic forces (natural selection) 4

5 Speciation by polyploidy hromosome doubling by autopolyploidy or allopolyploidy Reproductive isolation by selection or drift? Drosophila pseudoobscura 4 populations raised on starch-rich diet: Ist, IIst, IIIst, IVst This type of speciation is rapid 4 populations raised on maltose-rich diet: Ima, IIma, IIIma, IVma Whether new polyploid species survives is stochastic. One year of adaptation to diet like mates with like Needs to establish local population not swamped by parental gametes olyploidy very common: 40-70% of plant species Mate choice experiments: there is assortative mating? Selection and NOT drift causes assortative mating Dodd (1988) Evolution Speciation by drift + selection Hawaiian Drosophila radiation New species clearly due to founders from older islands But NO evidence for low genetic diversity in these species Disruptive selection Selection against intermediates Disruptive selection + assortative mating can produce a bimodal distribution robably important for sympatric speciation Speciation on new islands due to selection in new environment NOT due to drift in founders 5

6 Host races Rhagoletis pomonella Native host: hawthorn Became apple pest in 1860s, due to a host switch Apple-eating form quickly spread all over E. USA Speciation in cichlids Sympatric speciation due to disruptive selection leading to ecological specialisation? 1. arasitoids less successful with apple larvae (selective advantage of host switch) 2. Females have genetic preference for laying on fruits of own host (disruptive selection on host choice) 3. Males use host fruits as mating venue. So host switch has a pleiotropic effect on assortative mating 4. Apple race flies earlier than hawthorn race. leiotropy again 5. Leading to genetic differentiation Speciation in cichlids: sexual selection Sexual dimorphism: males often brightly coloured losely related species often different colours Assortative mating: female preference based on male colour Random mating when no colour information (e.g. when water is turbid). Reinforcement Reinforcement: the enhancement of prezygotic isolation in sympatry by natural selection Divergent forms meet in secondary contact Random mating may now create unfit hybrids Hybridization opposed by natural selection. Direct selection for assortative mating (Dobzhansky 1940) Adaptive mate choice, now termed reinforcement. Seehausen et al. (1997) Science 277: rediction: remating isolation stronger in sympatric compared to allopatric populations. 6

7 Reinforcement in Drosophila oyne & Orr (1997) Evolution 51: Reinforcement in Drosophila oyne & Orr (1997) Evolution 51: pairs of closely related divergent forms. re-mating isolation = fraction of trials of males and females of two species resulting in mating. ost-mating isolation = fraction of crosses of males and females of two species in which hybrids sterile or inviable. re-mating isolation in sympatric pairs > allopatric pairs ost-mating isolation in sympatric pairs allopatric pairs Sympatrc arapatric Allopatric Brief summary Geographic modes of speciation Status Known, but good examples are rare Known, likely Little different in theory from allopatric Known. Slow? Random Selection Random + selection auses of speciation Mutation + drift hromosomal mutation (polyploidy) Environmental, pleiotropic and disruptive Reinforcement Sexual selection Founder effect Shifting balance Brief summary Deeply suspect! If occurs, probably very slow, needs allopatry Known Known Known. ossibly uncommon? Suspected Status Dubious, contested ossible, contested 7

8 Further Reading FUTUYMA, DJ Evolutionary Biology. hapter 16 (pp ). Speciation. BARTON, NH (ed.): Trends in Ecology and Evolution, Speciation special issue, July OYNE, JA & ORR, HA Speciation. Sinauer Associates, Sunderland, Mass. xiii+545 pages. 8